1. Field of the Invention
The present invention relates to a barcode scanner and more specifically, to an optical system for barcode scanner, which enhances the brightness on the left and right sides of the image mapping surface on the barcode so that the lens can pick up a clear and bright barcode image.
2. Description of the Related Art
In the modern multimedia era, information exchange among people is popular, and the demand of people for information is more and more heavy. Rising of the new markets and fast developing and application of new technology bring prosperity for optical input devices. For the sake of commercial and industrial automation, barcodes have been intensively used to label a variety of products, tools and equipments, and barcode reading devices, i.e., barcode scanners are used to read barcodes on these products, tools and equipments by means of approaching the machine to the object or the object to the machine.
FIGS. 5 and 6 illustrates the light emitting device and lens of an optical system for barcode scanner according to the prior art. When the light source A is emitting light onto the barcode (not shown), the center area of the barcode image has a relatively higher brightness and the two opposite lateral areas of the barcode image have a relatively lower brightness. This uneven brightness of the barcode image may produce a weak image signal, resulting in a reading error or failure. Further, when the lens B picks up the barcode image, the receiving characteristic of relatively wider center receiving area and relatively narrower lateral receiving area of the lens makes the aforesaid unevenly distributed image brightness problem worse. In order to eliminate this problem, a light shade film B1 may be attached to the surface of the lens B to shade a part of the center area so that the lens B can obtain a barcode image having an evenly distributed brightness for accurate barcode reading. However, the image obtained in this light shading manner is relatively darkened, and the aforesaid reading error or failure problem may still occur. Further, the light source A will have a light attenuation problem after a long use. When the light source A starts to attenuate, the barcode image obtained by the lens B will be more darkened and unreadable, thereby shortening the working life of the barcode scanner and increasing the user's cost of use.
FIG. 7 shows another design of optical system used in a barcode scanner according to the prior art. According to this design, the light emitting direction of the light source A in the barcode scanner C is not in parallel to the scanning direction of the lens B but at an angle θ about 15°. Because the light emitting direction of the light source A and the scanning direction of the lens B are not in parallel, a part of light rays is dispersed when the light source A is emitting light onto the barcode. Therefore, the light source A must use multiple rows of light emitting devices to light a big area so as to obtain an even brightness on the X-Y plane. Further, when scanning, the light source A must be kept in proximity to the barcode. The precision arrangement of the multiple rows of light emitting devices greatly increases the cost of the barcode scanner.
FIGS. 8 and 9 show still another design of optical system for barcode scanner according to the prior art. According to this design, a diffraction grating D is used to diffuse light evenly. This arrangement can only distribute light evenly. Further, a certain distance must be kept between the condenser E and the light source A to obtain a satisfactory condensing effect. This arrangement requires much inside space of the scanner, i.e., the barcode scanner cannot be small-sized. Further, the use of multiple sets of light emitting devices for the light source A also increases the cost of the barcode scanner.
Therefore, it is desirable to provide an optical system for barcode scanner that eliminates the aforesaid problems.